The island is about 4.5 km long & 2 km wide. The island is flat and low-lying.

Except for the eastern side, Dassen Island is fringed with reefs & many ships have run aground here. It is composed of a fine-grained tourmaline-granite (with a few areas of biotite granite) outcrop covered with sand. Some of the granites along the shore, to just above the high-water mark, consists of large, rounded boulders which stick out above the sand.

Though in the winter temporary pans forms in the interior, there is generally little fresh water on the island.